The development of superconducting electrical generators has led to the use of airgap armature windings. These airgap armature windings are distinguishable from conventional stator windings by their structure which includes a support mechanism which does not include insertion in a slotted core. Conventional stators employ a generally rigid cylindrical core which is constructed from a plurality of stacked laminations. Each lamination has a plurality of teeth which, when stacked together, result in axial grooves in the inner cylindrical periphery of the core. By inserting the stator windings within these grooves, support is provided for the stator coils.
In contradistinction to the above described conventionally constructed stator, superconducting generator stators support the stator winding in generally rigid nonconductive material which is typically cylindrically shaped and which encase the conductive stator coils. This material typically has less mechanical strength than that provided by the laminated core and, therefore, the stresses to which the coils are subjected must be held to values within this lesser strength.
There are two major sources of mechanical stress in a superconducting generator stator, one due to thermal expansion and another caused by electromagnetic forces which occur during transient faults. When a transient fault occurs following a sustained operation which has produced thermal expansion of the stator components, the total stresses on the stator structure could be approximately equivalent to the sum of these two conditions and could approach or exceed the strength of the materials used to support the airgap armature winding.
Since the materials which are presently used in the support structure of airgap armature windings are of limited strength, a means for minimizing the stresses on them is required. The present invention provides a way of avoiding the additive stresses, particularly their radial components, resulting from both the thermal expansion and transient fault forces described above.
A generator stator made in accordance with the present invention incorporates an outer support structure around the airgap armature structure. Both are generally cylindrical and are associated in a coaxial and concentric assembly. A gap, or interface space, is provided between these two members which is generally cylindrical. Grooves are shaped in both the inner surface of the outer support structure and the outer surface of the airgap armature structure. These grooves are aligned to form a channel which is intersected by the above mentioned interface gap.
Within the channel, an interface support structure is disposed. It consists of a porous bar which is impregnated with a liquid, such as transformer oil, and encapsulated by an impermeable covering, such as plastic. Although a single interface support structure is described herein, it should be understood that a plurality of interface structures may be used, each being disposed in a separate channel.
The porous bar, which can be a fiber composite, is deformable under a sustained force but provides a rigid support capable of withstanding sudden forces of short duration, such as those caused by transient faults. The bar's ability to withstand short duration impulses enables the support system to provide stiffness which resists sudden deflections whereas the bar's gradual deformation under long duration forces, such as those caused by thermal expansion, enables it to avoid built up stresses caused by this expansion.
Since transient faults in an electrical generator typically produce forces which are tangential to the stator coil structure, the present invention is most advantageously positioned to extend axially along the interface gap. However, it should be understood that the porous bar of the present invention can be configured in any direction which extends perpendicular to the direction of anticipated forces.